A “speaker” generates sound from an electrical signal. In the hearing aid art, one often encounters the term “receiver” for such a device, which reads strangely to the uninitiated. “Electroacoustic transducer” is clumsy and pedantic. Thus, “speaker” is the term used for describing this invention.
A human ear canal is a narrow, irregular, tubular structure, approximately 25 mm in length. Coupling amplified sound to the eardrum at the inner end of the canal is not as simple as it might seem. In a hearing aid, a microphone is connected to a speaker by a high gain (60-80 dB) amplifier and is relatively close to the speaker, 1-5 cm.
If an acoustic path exists between the speaker and the microphone, sound from the speaker feeds back to the microphone. Feedback typically occurs at high frequencies due to the higher gain at these frequencies, where most hearing loss occurs. Technically, when the output from the speaker is coupled in phase to a microphone and the loop gain exceeds unity, there is feedback. Feedback is a sharp tonal sound, often at the higher frequencies.
Feedback manifests itself as an unpleasant squeal that quickly grows in magnitude until maximum amplification is reached. The squeal can be audible even to those several feet from the hearing aid. Feedback can be eliminated by reducing the gain of the amplifier by way of a volume control on the hearing aid. Often the wearer is obliged to adjust the gain frequently as the loudness of background sounds and the loudness of sounds of interest change. Feedback in a hearing aid can interfere with hearing and may cause the wearer not to use the hearing aid. High level feedback in a hearing aid may even damage the already impaired hearing of the wearer.
In most modern hearing aids, an adaptive feedback canceller is used to cancel feedback. A digital filter continuously models the feedback path and generates an estimate of the feedback signal. This estimate is subtracted from the incoming signal to provide feedback cancellation. Constraints must be imposed to ensure that audio quality is not sacrificed when the hearing aid is presented with tones or narrow band sounds that cause the digital filter to converge to incorrect values. While adaptive feedback cancellation is effective to a degree, false positives are a constant problem. Eliminating a tone when a user is listening to music, for example, can cause great frustration.
Eliminating feedback involves reducing loop gain at a particular frequency using a notch filter or reducing loop gain in a narrow band of frequencies using less selective filters. Using notch filters to reduce gain often requires computational resources that are inherently limited by the size and power consumption of the semiconductor chip implementing the filters, which is limited by the size of the hearing aid.
Hearing aids can be divided into four groups: Behind-The-Ear (BTE), In-The-Ear (ITE), In-The-canal (ITC), and Completely-In-the-Canal (CIC). Some BTE hearing aids have an advantage over other types because the speaker is relatively far from any microphone in the body of the hearing aid. This invention is applicable to all types of hearing aids and to other applications where acoustic feedback is a problem.
As is well known to those of skill in the art, once an analog signal is converted to digital form, all subsequent operations can take place in one or more suitably programmed microprocessors. Special purpose circuits can be used instead of general purpose circuits for improved efficiency or reduced cost. Reference to “signal,” for example, does not require nor exclude a particular implementation and can be analog or digital. Data in memory, even a single bit, can be a signal. In other words, a block diagram can be interpreted as hardware, software, e.g. a flow chart or an algorithm, or a mixture of hardware and software. Programming a microprocessor and other devices is well within the ability of those of ordinary skill in the art, either individually or in groups.
In view of the foregoing, it is therefore an object of the invention to provide a system for managing acoustic feedback.
Another object of the invention is to eliminate or minimize false positives in an adaptive feedback cancellation system.
A further object of the invention is to provide improved feedback detection.
Another object of the invention is to accurately minimize the gain of a feedback signal.
A further object of the invention is to cancel feedback that cannot be handled by an adaptive feedback canceller within its prescribed limits.
Another object of the invention is to provide effective and rapid cancellation of feedback at more than one frequency.
A further object of the invention is to prevent a loss of gain at frequencies where feedback is not occurring even while feedback is being cancelled at one or more other frequencies.
Another object of the invention is to cause negligible degradation of audio quality while reducing feedback.
A further object of the invention is to handle varying amounts of feedback including catastrophic situations.
Another object of the invention is to provide feedback management with maximum use of available resources such as adaptive filters.